Characterization and evaluation method for macrosegregation of alloy elements in ferritic stainless steel

Abstract

The invention discloses a characterization and evaluation method for macrosegregation of alloy elements in ferritic stainless steel, which comprises the following steps: selecting a proper heat treatment process according to a Laves phase solid solution temperature calculated by thermodynamic software, and carrying out heat treatment on a ferritic stainless steel plate; determining whether a banded structure, namely macrosegregation, exists or not through a metallographic microscope; and an electronic probe is adopted to test the segregation degree of alloy elements in the ferritic stainless steel. The method is simple and easy to implement, and has definite guiding significance on the actual service behavior of the ferritic stainless steel in the high-temperature field.

Description

Technical Field

[0001] The present invention relates to the field of stainless steel research and detection, and in particular to a method for characterizing and evaluating the macrosegregation of alloying elements in ferritic stainless steel. Background Art

[0002] Ferritic stainless steel has a body-centered cubic structure and has the characteristics of high strength, low tendency to cold work hardening, high thermal conductivity, small expansion coefficient, and good oxidation resistance. It is widely used in high-temperature fields such as automotive exhaust systems, heat exchange pipes, and high-temperature pressure vessels.

[0003] To adapt to high-temperature service conditions, ferritic stainless steel is added with alloying elements such as niobium, molybdenum, and tungsten, either singly or in combination. This can significantly improve the high-temperature strength of stainless steel through two mechanisms: solid solution strengthening and precipitation strengthening. Solid solution strengthening is the process of alloying elements dissolving in the base metal, causing a certain degree of lattice distortion, thereby increasing the strength of the alloy. Niobium, molybdenum, and tungsten have large atomic sizes and exhibit excellent strengthening effects. Precipitation strengthening is the process of second-phase particles precipitating from the matrix, which are dispersed throughout the matrix and hinder dislocation motion, thereby strengthening the material. In ferritic stainless steel, the Laves phase—Fe2(Nb,Mo,W)—is the primary precipitation strengthening phase at service temperatures.

[0004] The production process of ferritic stainless steel plate is as follows: smelting - continuous casting - hot rolling - hot plate annealing and pickling - cold rolling - cold plate annealing and pickling. During the initial solidification phase of the continuous cast ingot, fine equiaxed crystals form on the surface, followed by columnar crystals that begin to form inward. In the core of the ingot, due to the decreasing temperature gradient, a central equiaxed crystal zone forms. Due to the rapid growth rate of columnar crystals, niobium, molybdenum (positively segregating elements), and tungsten (negatively segregating elements) are prone to macrosegregation in ferritic stainless steel. Macrosegregation is hereditary, passing from the ingot to the hot-rolled plate and the finished cold-rolled product. This manifests as an uneven microstructure in the steel plate, characterized by the presence of banded structures formed by precipitated phases in the segregated areas.

[0005] In current industrial production, stainless steel plates are annealed at high temperatures for short periods of time to improve production efficiency. This process temporarily dissolves the precipitated phases in the segregated areas back into the matrix, masking the elemental segregation. However, macrosegregation is not eliminated. Once the steel plate is exposed to high temperatures, the secondary phase in the segregated areas will exhibit a greater tendency to precipitate, thus affecting high-temperature performance.

[0006] To ensure the performance of ferritic stainless steel, it is necessary to test for the presence and extent of macrosegregation of alloying elements. However, in actual production and application, macrosegregation testing is performed on continuously cast billets. The specific method involves sampling from different parts of the billet and performing chemical composition analysis to determine the distribution of various elements. This method can accurately analyze the segregated elements and their extent, but the testing process is complex and time-consuming.

[0007] In summary, a simple and easy method to characterize and evaluate the macrosegregation of alloying elements in ferritic stainless steel is needed.

[0008] The present invention provides a method for characterizing and evaluating the macrosegregation of alloying elements in ferritic stainless steel. This method can directly characterize the presence of macrosegregation of alloying elements in ferritic stainless steel sheets and evaluate the degree of segregation. Compared with existing methods, this method eliminates the need for multiple samples from the continuously cast slab and subsequent testing. The method is simple and easy to use, and the results are clear and concise, providing clear guidance for the actual service behavior of ferritic stainless steel in high-temperature applications. Summary of the Invention

[0009] The purpose of the present invention is to address the above problems and provide a method for characterizing and evaluating the macrosegregation of alloying elements in ferritic stainless steel.

[0010] The object of the present invention is achieved as follows: a method for characterizing and evaluating the macrosegregation of alloying elements in ferritic stainless steel comprises the following steps: Step 1: Calculating the thermodynamic equilibrium phase diagram of ferritic stainless steel by thermodynamic calculation software to obtain the solid solution temperature of the Laves phase T ; Step 2: Heat treat the ferritic stainless steel plate at a temperature range of [ T +(30~100)]℃, time is 5~10min / mm; Step 3: Polish the cross section of the heat-treated stainless steel plate to a mirror surface, and observe the surface under a metallographic microscope after corrosion with hydrochloric acid and ferric chloride aqueous solution. If there is no banded structure, there is no macrosegregation. If there is a banded structure, there is macrosegregation. Mark the position of the banded structure, that is, the distance between the banded structure and the edge of the sample; Step 4: Mechanically polish the surface of the sample with macrosegregation to a mirror surface again without corrosion, and use an electron probe to test the segregation degree of alloying elements in ferritic stainless steel: First, locate the position of the sample banded structure, determine the precipitation of the second phase at this position in the backscattering mode, and measure the width of the banded structure; Second, use 1 / 2 of the width of the banded structure as the measuring diameter, test the chemical composition at this location, and obtain the content of the alloying elements. C 偏析 ; Third, the composition test is carried out on the substrate with the same measurement diameter to obtain the alloy element content C 基体 ; Calculate the degree of segregation of alloy elements by the formula: K= C 偏析 / C 基体 .

[0011] The chemical composition of ferritic stainless steel is: C≤0.02%, N≤0.02%, Si: 0.1~0.5%, Mn: 0.1~1.0%, Cr: 11~30%, Nb: 0.2~1.0%, Mo: 0~4.0%, and (Nb+Mo)≥0.4%, the rest is Fe and other inevitable impurity elements.

[0012] The thickness of ferritic stainless steel plate is 0.3~20.0mm.

[0013] The present invention provides a method for characterizing and evaluating the macrosegregation of alloying elements in ferritic stainless steel. This method can directly characterize the presence of macrosegregation of alloying elements and evaluate the degree of segregation in ferritic stainless steel sheets. Compared with existing methods, this method eliminates the need for multiple samples from the continuously cast slab and subsequent testing. The method is simple and easy to use, and the results are clear and concise, providing clear guidance for the actual service behavior of ferritic stainless steel in high-temperature applications. BRIEF DESCRIPTION OF THE DRAWINGS

[0014] The present invention will be further described below in conjunction with the accompanying drawings.

[0015] Figure 1 This is the microstructure diagram of the ferritic stainless steel plate after heat treatment in Example 1.

[0016] Figure 2 This is the microstructure diagram of the ferritic stainless steel plate after heat treatment in Example 2.

[0017] Figure 3 This is the microstructure diagram of the ferritic stainless steel plate after heat treatment in Example 3.

[0018] Figure 4 This is the microstructure diagram of the ferritic stainless steel plate after heat treatment in Example 4. DETAILED DESCRIPTION

[0019] In response to the above problems, the present invention provides a method for characterizing and evaluating the macrosegregation of alloying elements in ferritic stainless steel. The technical solution is carried out according to the following steps: using thermodynamic calculation software such as Thermo-calc, the thermodynamic equilibrium phase diagram of ferritic stainless steel is calculated to obtain the solid solution temperature of the Laves phase. T ; The ferritic stainless steel plate is heat treated in the temperature range of [ T +(30~100)]℃, time is (5~10)min / mm; if there is no macrosegregation in ferritic stainless steel, the steel plate is heat treated above the Laves phase solution temperature (the actual production experience value is about ( TAt temperatures above +20°C, no Laves phase precipitates in the matrix. However, if macrosegregation occurs, alloying elements are concentrated in the segregated areas. The high alloy content in these areas increases the solution temperature of the Laves phase, preventing the Laves phase from fully dissolving during heat treatment within a certain temperature range, thus forming a banded structure. Therefore, the heat treatment temperature is determined to ensure that the Laves phase in the matrix is fully dissolved while remaining partially dissolved in the segregated areas.

[0020] The cross section of the heat-treated ferritic stainless steel plate is polished to a mirror surface and observed under a metallographic microscope after surface corrosion. If there is no banded structure, there is no macrosegregation. If there is, the position of the banded structure is marked (i.e., the distance between the banded structure and the edge of the sample). The surface of the sample with macrosegregation is mechanically polished again without corrosion. The degree of segregation of alloying elements in ferritic stainless steel is tested using an electron probe: first, locate the position of the sample banded structure, determine the presence of a second phase precipitation at this position in the backscattering mode, and measure the width of the banded structure; second, use 1 / 2 of the banded structure width as the measurement diameter, test the chemical composition at this location, and obtain the content of alloying elements. C 偏析 ; Third, the composition test is carried out on the substrate with the same measurement diameter to obtain the alloy element content C 基体 .

[0021] The segregation degree of alloying elements is calculated by the formula: K= C 偏析 / C 基体 .

[0022] The chemical composition of ferritic stainless steel is: C ≤0.02%, N ≤0.02%, Si: 0.1~0.5%, Mn: 0.1~1.0%, Cr: 11~30%, Nb: 0.2~1.0%, Mo: 0~4.0%, W: 0~4.0%, and (Nb+Mo) ≥0.4%, the rest is Fe and other inevitable impurity elements.

[0023] The thickness of ferritic stainless steel plate is 0.3~20.0mm.

[0024] The method provided by the present invention can quickly determine whether ferritic stainless steel has macrosegregation and evaluate the degree of segregation.

[0025] The characterization and evaluation method of macrosegregation of alloying elements in ferritic stainless steel provided by the present invention are described in detail below with reference to the examples, but the specific embodiments of the present invention are not limited to the following examples. Example 1

[0026] This example characterizes and evaluates the macrosegregation of alloying elements in a ferritic stainless steel plate with a thickness of 5.0 mm.

[0027] Its chemical composition by mass percentage is: C: 0.009%; N: 0.008%; Si: 0.34%; Mn: 0.15%; P: 0.015%; S: 0.001%; Cr: 18.11%; Nb: 0.41%; Ti: 0.18%. The rest is Fe and unavoidable impurities.

[0028] Thermo-calc software was used to determine the solid solution temperature of the Laves phase to be 930°C.

[0029] When heat treating ferritic stainless steel plates, the temperature is 1000°C and the holding time is 30 minutes.

[0030] The cross section of the heat-treated stainless steel plate was polished to a mirror surface, and the surface was corroded and observed under a metallographic microscope. The microstructure is as follows: Figure 1 As shown. Figure 1 It can be seen that there is a banded structure in this sample, indicating the presence of macrosegregation. The banded structure is measured to be approximately 1.3 mm from one edge of the sample.

[0031] The sample surface was mechanically polished again without corrosion, and placed in the electron probe sample chamber. The sample was positioned at the location of the banded structure. The second phase was determined to be precipitated at this location under the backscattering mode. The width of the banded structure was measured to be approximately 100 mm. The measurement diameter is 50 mm, which is half of the width of the banded structure. The chemical composition of this area is tested to obtain the content of the alloying element Nb. C 偏析 ; Carry out composition test on the matrix with the same measurement diameter to obtain the Nb content C 基体 The segregation degree of the positive segregation element Nb is calculated by the formula: K= C 偏析 / C 基体 The results are shown in Table 1.

[0032] Example 2

[0033] This example characterizes and evaluates the macrosegregation of alloying elements in a ferritic stainless steel plate with a thickness of 3.0 mm.

[0034] Its chemical composition by mass percentage is: C: 0.012%; N: 0.010%; Si: 0.25%; Mn: 0.22%; P: 0.012%; S: 0.001%; Cr: 23.11%; Nb: 0.56%; W: 1.94%. The rest is Fe and unavoidable impurities.

[0035] Using Thermo-calc software, the solid solution temperature of Laves phase was found to be 990℃.

[0036] When heat treating ferritic stainless steel plates, the temperature is 1040°C and the holding time is 25 minutes.

[0037] The cross section of the heat-treated stainless steel plate was polished to a mirror surface, and the surface was corroded and observed under a metallographic microscope. The microstructure is as follows: Figure 2 As shown. Figure 2 It can be seen that there is a banded structure in this sample, indicating the presence of macrosegregation. The banded structure is measured to be approximately 0.8 mm from one edge of the sample.

[0038] The sample surface was mechanically polished again without corrosion, and placed in the electron probe sample chamber. The sample was positioned at the location of the banded structure. The second phase was determined to be precipitated at this location under the backscattering mode. The width of the banded structure was measured to be approximately 200 mm. The measurement diameter is 1 / 2 of the width of the banded structure, i.e. 100 mm. The chemical composition of this area is tested to obtain the content of alloying elements Nb and W. C 偏析 ; Using the same measurement diameter to test the composition of the matrix, the content of W was obtained. C 基体 The segregation degree of alloying elements Nb and W is calculated by the formula: K= C 偏析 / C 基体 The results are shown in Table 2.

[0039] Example 3

[0040] In this example, the macrosegregation of alloying elements in a ferritic stainless steel plate with a thickness of 12.0 mm was characterized and evaluated.

[0041] Its chemical composition by mass percentage is: C: 0.010%; N: 0.009%; Si: 0.35%; Mn: 0.19%; P: 0.012%; S: 0.001%; Cr: 18.05%; Mo: 2.01%; Nb: 0.20%. The rest is Fe and unavoidable impurities.

[0042] Thermo-calc software was used to determine the solid solution temperature of the Laves phase to be 860°C.

[0043] When heat treating ferritic stainless steel plates, the temperature is 900°C and the holding time is 60 minutes.

[0044] The cross section of the heat-treated stainless steel plate was polished to a mirror surface, and the surface was corroded and observed under a metallographic microscope. The microstructure is as follows: Figure 3 As shown. Figure 3 It can be seen that there is no banded structure in this sample, and therefore no macrosegregation. Example 4

[0045] This example characterizes and evaluates the macrosegregation of alloying elements in a ferritic stainless steel plate with a thickness of 1.5 mm.

[0046] Its chemical composition by mass percentage is: C: 0.008%; N: 0.011%; Si: 0.40%; Mn: 0.80%; P: 0.02%; S: 0.002%; Cr: 18.05%; Mo: 1.97%; Nb: 0.55%; W: 1.02%. The rest is Fe and unavoidable impurities.

[0047] Using Thermo-calc software, it was found that the solid solution temperature of the Laves phase was 1000℃.

[0048] When heat treating ferritic stainless steel plates, the temperature is 1040°C and the holding time is 10 minutes.

[0049] The cross section of the heat-treated stainless steel plate was polished to a mirror surface, and the surface was corroded and observed under a metallographic microscope. The microstructure is as follows: Figure 4 As shown. Figure 4 It can be seen that there are multiple bands in this sample, indicating macrosegregation. The bands are measured to be approximately 0.5 mm from one edge of the sample.

[0050] The sample surface was mechanically polished again without corrosion, and placed in the electron probe sample chamber. The sample was positioned at the location of the banded structure. Under the backscattering mode, it was determined that the second phase precipitated at this location. The width of a more serious banded structure was measured to be about 100 mm. The measurement diameter is 50 mm, which is half of the width of the banded structure. The chemical composition of this area is tested to obtain the content of alloying elements Nb, W, and Mo. C 偏析 ; Carry out composition test on the substrate with the same measurement diameter to obtain the content of alloy elements Nb, W, and Mo C 基体 .

[0051] The segregation degree of alloying elements Nb, W, and Mo is calculated by the formula: K= C 偏析 / C 基体 The results are shown in Table 3.

[0052]

[0053] The above description is only a specific embodiment of the present invention, but the structural features protected by the present invention are not limited thereto. Any changes or modifications made by any technician in this field within the scope of the present invention are included in the patent scope of the present invention.

Claims

1. Characterization and evaluation method of macrosegregation of alloying elements in ferritic stainless steel, characterized by: The following steps are involved: Step 1: Calculate the thermodynamic equilibrium phase diagram of ferritic stainless steel using thermodynamic calculation software to obtain the solid solution temperature of the Laves phase T ; Step 2: Heat treat the ferritic stainless steel plate at a temperature range of [ T +(30~100)]℃, time is 5~10min / mm; Step 3: Polish the cross section of the heat-treated stainless steel plate to a mirror finish. After etching the surface with hydrochloric acid and ferric chloride aqueous solution, observe it under a metallographic microscope. If there is no banded structure, there is no macrosegregation. If there is banded structure, there is macrosegregation. Mark the position of the banded structure, that is, the distance from the banded structure to the edge of the sample. Step 4: The surface of the sample with macro-segregation is mechanically polished to a mirror surface again without corrosion, and the degree of segregation of alloy elements in ferritic stainless steel is tested by electron probe: First, locate the position of the sample banded structure, determine the second phase precipitation at this position in the backscattering mode, and measure the width of the banded structure; second, use 1 / 2 of the banded structure width as the measurement diameter, test the chemical composition at this position, and obtain the content of alloy elements. C 偏析 ; Third, the composition test is carried out on the substrate with the same measurement diameter to obtain the alloy element content C 基体 ; Calculate the degree of segregation of alloy elements by the formula: K= C 偏析 / C 基体 .

2. The method for characterizing and evaluating macrosegregation of alloying elements in ferritic stainless steel according to claim 1, characterized in that: The chemical composition of ferritic stainless steel is: C≤0.02%, N≤0.02%, Si: 0.1~0.5%, Mn: 0.1~1.0%, Cr: 11~30%, Nb: 0.2~1.0%, Mo: 0~4.0%, and (Nb+Mo)≥0.4%, the rest is Fe and other inevitable impurity elements.

3. The method for characterizing and evaluating macrosegregation of alloying elements in ferritic stainless steel according to claim 1, wherein: The thickness of ferritic stainless steel plate is 0.3~20.0mm.

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